the Creative Commons Attribution 4.0 License.
the Creative Commons Attribution 4.0 License.
| 29 Jul 2025
Volume-to-extinction ratio: An important property of dust
Abstract. The volume-to-extinction ratio (ζ) is an important aerosol property, allowing to relay gravimetric and optical quantifications, widely used in remote sensing and in climate models. The ζ parameter is affected by the microphysical properties of aerosol particles, including their size, shape and composition . This study presents a novel, synergistic approach combining airborne in-situ observations and ground-based remote sensing to study this parameter during dust events originating in the Middle East and Saharan regions, and examine its variability and general estimation uncertainty. The data were collected during the 2021 Cyprus Fall Campaign and the 2022 ASKOS campaign in Cabo Verde. The combination of observations offered vertically-resolved observations of the particle size-distribution and volume-to-extinction ratio. The findings of this study reveal significant variability in the ζ parameter and the effective radius across different events and regions. During Middle East dust events in Cyprus the observed average ζ was the lowest with ζ = 0.53 ± 0.24 μm, whilst for a Saharan dust case in Cabo Verde observations showed the highest values with ζ = 1.14 ± 1.01 μm, both at the dust layer altitude. The analysis highlights large discrepancies compared to AERONET-derived values and previous literature, especially in the presence of coarse and giant particles. Scattering computations allowed to evaluate the experimental results and provide insights into the role of particle asphericity. Atmospheric model simulations also showed discrepancies, mainly due to assumptions that neglect larger particles. These findings suggest that improve dust representation in models is essential for accurate climate assessment.
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RC1: 'Comment on egusphere-2025-3404', Anonymous Referee #1, 17 Aug 2025
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This manuscript addresses one of the most relevant topics in the study of desert dust, which is the need to better characterise coarse and super-coarse particles in the atmospheric load. Using data collected during two campaigns (the 2021 Cyprus Fall Campaign and the 2022 ASKOS campaign in Cape Verde), the authors aim to demonstrate that ground based remote sensing instruments, such as the AERONET sun-photometers, have limitations in their products when it comes to particles larger than 15 μm, leading to an underestimation in the number of these particles. They also show the relevance of parameters such as the shape of these particles in simulating their properties. To this end, the authors focus on comparing the volume-extinction ratio obtained from their observations with the value that can be determined from AERONET products or simulation algorithms. The observed values are based on extinction profiles obtained from lidar measurements and volume concentrations from particle counters installed on UAVs.
The topics covered in this study are highly relevant and fall within the scope of this journal. The structure is clear and logical, and the language is formal and easy to understand. The methods and selection data criteria used are described in detail, and the estimated uncertainties are well justified. The figures and tables are presented clearly and are relevant for illustrating the results. The research is original, as few campaigns involving UAVs have been carried out, and the results are compared with the literature. In addition, the article includes an adequate number of references and the dataset used is provided. The title is appropriate and clearly represents the objective of the study. The abstract accurately describes the research conducted and the relevance of the results obtained. The conclusion summarises the methodology, discussion and reinforces the relevance of the results. Therefore, I strongly recommend the publication of this article after some minor revisions are addressed.
Specific comments:
- In Figure 1, four values of the sphericity are indicated, but only three colour intensities can been distinguished.
- Introduction: Do you have any information about the error in the retrieved parameters from AERONET (AOD, AE, volume size distribution and refractive index)? What data level is used in the analysis?
- Sections 3.3.1. and 3.3.2: It might be better to indicate the name of the instrument in the section title and use the abbreviation from there on (e.g. 3.3.1. Portable Optical Particle Spectrometer (POPS).).
- Section 3.3.3: Is the sensitivity of the impactors known?
- It might be better to group sections 3.4 and 3.5. under a new subsection 3.4. focused on simulation algorithms.
- Line 276: What lidar wavelength is used?
- Line 277: Unfinished sentence.
- Section 3.5, line 295: Why is that specific configuration selected?
- Table 1: please indicate the meaning of the acronyms (VLDR, AOD NIC// AMX, OSCM) in the table caption.
- Section 4.2.1, line 331. What AOD value is chosen as the constraint?
- Figure 2: Why are not UCASS and POPS landings indicated?
- Figure 4a: The curve representing the mean values is difficult to distinguish. I suggest plotting the other curves with dashed lines.
- Figure 5a: Why are there red points if RH limit is not exceeded?
- Section 4.3.3.: How are the data points fitted in the intervals where measurements are taken from both instruments?
- Figure 10: legend, please include WRF-Chem
Some typos:
- Line 1: "relay", maybe relate?
- Lines 6-7: "observations" is repeated
- Line 61: perhaps include a parenthesis for the references?
- Line 92: two uses of e.g.
- Line 97: satellite "images"?
- Line 102: "outlines"
- Line 149: Eq. 17 from section 4.3.3.
- Line 208: "Raman"
- Line 254: u (m/s)
- Line 262: Scanning Electron Microscope (SEM)
- Line 274: aspect ratio (AR)
- Line 286: radius between 0.1 to...
- Line 288: Cyprus Institute (CYI)
- Line 407: Refractive index (RI)
- Line 502: theoretical
- Figure 11: first line of caption, please remove one in from "in in Table 1"
Citation: https://doi.org/10.5194/egusphere-2025-3404-RC1 -
CC1: 'Comment on egusphere-2025-3404', Albert Ansmann, 27 Aug 2025
reply
From the papers of Ryder et al. (2018) and Weinzierl et al. (BAMS 2017) we know already that large mineral dust particles with diameters larger than 30 micrometer can survive over several days of long range transport before they are removed by sedimentation processes.
On the other hand, we know also that the AERONET inversion scheme, used to analyze multiwavelength AOD and sky radiance data, to retrieve microphysical properties (including volume concentration) considers particle diameters up to 30 micrometer only so that volume-to-extinction ratios derived from AERONET observations may be wrong in the special situations, e.g. when analyzing lidar observations of optical dust properties close to the dust source region.
The question is now how relevant is this effect and what can we learn from the study presented in this manuscript, suggesting that a systematic underestimations of dust mass concentrations occurs when using the volume-to-extinction ratios derived from AERONET observations?
My review here is motivated by a number of not well-discussed points and partly not well-explained aspects. I miss for example a more critical discussion of the airborne (UAV) measurements by the authors, especially regarding the limits of the applied methods to obtain the volume size distribution of dust particles when there are giant dust particles with diameters above 30-40 micrometer in the air.
Page 3, Line 70: The authors write: UAVs can provide high vertical-resolution profiles of aerosol properties for fine, coarse, and giant particles. My question: How well can particles with diameters of 20,30, and 40 micrometer be counted with the UCASS? A bias in counting, i.e, wrong counting of the few large to giant particles has a huge effect on the derived dust volume concentration and can dominate the volume concentration retrieval completely.
In the paper of Kezoudi et al. (ACP, 2021), a strong bias is shown (in Figure 9) between the retrieved dust extinction coefficient (from the UAV in situ observations of the size distribution) and the directly measured dust extinction coefficient (from Raman lidar) in pronounced dust layers. The extinction values from UCASS were systematically too large pointing to too large counts (regarding large to giant particles), in most case by a factor of 2. And the volume concentration derived from such wrong counts or particle numbers (or extinction coefficients) would be ‘wrong’ by a factor of 2 as well, I conclude.
Please, expand the discussion on this problem!
During the ASKOS campaign a strong dust outbreak was studied (Cabo Verde, 24 June 2022). The Angstroem exponent was around 0.15. However, to my opinion, the detailed multiwavelength lidar observations (they are not presented in this paper!) do not support the presence of very large to giant dust particles in the observed dust layers, as derived from the UCASS observations!
I checked the Polly lidar data base. The lidar measured particle linear depolarization ratios (PLDRs) of 23-25% (355nm), close to 28-30% (532 nm), and 22-24% (1064nm) on 24 June 2022 over Cabo Verde. These values are in good agreement with typical PLDR values after long-range dust transport to Barbados (4000 km west of Africa. Haarig et al., ACP. 2017), or even to central Europe after fast long-range transport (two days from Sahara to Leipzig, Germany) as reported by Haarig et al. (ACP, 2021). The drop of the depolarization ratios from 30% at 532 nm to 22-24% at 1064nm does, to my opinion, not support the presence of large to giant dust particles, i.e., of particles with diameters larger than 10 micrometer. Burton et al. (ACP, 2015) found exceptionally high PLDRs of 30% (355nm) , 40% (532nm) , and 40% (1064nm) in just ONE unique dust plume (in a dust source region) with freshly emitted giant dust particles. Hu et al. (2022) measured 32% (355nm), 37% (532nm), 32% (1064nm) close to the Taklamakan desert. Even here, close to the Taklamakan source region, the very large and giant particles seem to fall out quickly.
To my opinion, the PLDR spectrum measured with lidar over Cabo Verde on 24 June 2022 was quite normal for aged dust plumes, pointing to dust particles at all smaller than 5 micrometer (in radius). These lidar observations are in agreement with the accompanying AERONET photometer observations (AODs, size distribution) at Cabo Verde, pointing to a dust effective radius around 2-3 micrometer. More details to AERONET observations are given below.
My rough computations indicate that particles with diameters of 20, 30, and 40 micrometer fall about 500m, 1500 m, and 3000 m per day, respectively, at tropospheric conditions below 4 km height. How many days did the dust travel from the source regions to Cabo Verde arriving on 24 June 2022 according to HYSPLIT backward trajectories? Probably more than three days? I miss trajectory studies and travel times in the manuscript.
Further points:
Page 5, In Eq. (7), the effective radius is 3V/4A! Maybe I missed the point! To my opinion, the effective radius is defined as 3V/A.
Page 9 241-242: POPS can provide reliable vertical profiles of the particle size distribution. Please provide uncertainty numbers already in this section!
Page 9, 249-250: UCASS airborne particle size distributions were in close agreement with other reference OPCs? Please provide clear uncertainty values already in this section?
Page 12, Table 1: Angstroem exponents of 0.76-1.17, i.e., clearly larger than 0.6, indicate a quite undefined mixture of dust and anthropogenic pollution. And the selected Cyprus case of 15 November showed Angstroem exponents around 1. At such conditions, pollution dominates the optical properties of the aerosol. What can we learn from UAV observations at these complicated conditions ... with respect to the dust volume-to-extinction relationship?
Page 13, Figure 3: One needs to show the PLDR instead of the VDR to get a good impression on the dust fraction. The Cyprus profiles tell us that the full layer from the surface to 2.5 km was well mixed and consists of dust and pollution aerosol, but the dust fraction remains unresolved when showing VDR.
Page 13, Figure 4(a): The Cyprus volume size distribution shows a pronounced fine mode (even in the volume size distribution). Thus, pollution dominates the optical properties. Figure 4(b) on the other hand (Cabo Verde, 24 June 2022) shows a typical dust size distribution with a typical ‘left wing’ of dust fine mode particles. The maximum of the coarse mode is at 2 micrometer, for me a reasonable value, and in full consistency with the observed Polly PLDR spectrum. AERONET observations in Tamanrasset (directly in the dust source region) often show maximum values around 5 micrometer. But here (24 June, Cabo Verde) such a maximum is not found, and thus giant particles were obviously absent according to the AERONET observations but derived from the in situ measurements. So, please expand the discussion on this point, especially regarding the limits of the UCASS observations expressed in terms of counting uncertainties for large to very large and giant particles.
Page 20, Figure 9: As a reader, I have just to accept (or believe) the strong deviation between the size distribution derived from AERONET observations and derived from in situ observations. I have no option to check how trustworthy the in situ observations are. It would be desirable (in future) to model the dust optical properties (especially the backscatter coefficients and depolarization ratios at 355, 532, and 1064nm) with sophisticated optical models that are able to properly consider the dust size and shape characteristics. But such models are not available yet.
Page 21: Figure 10(b) is then not a surprize … But is that the truth?
In conclusion, the message of the paper is clear. The study is worthwhile to be published! We have to be careful when converting dust optical properties into dust mass concentrations based on AERONET derived mass-to-extinction conversion factors! But how relevant is this effect? How often do such conditions occur? This important question remains open!
Citation: https://doi.org/10.5194/egusphere-2025-3404-CC1 -
RC2: 'Comment on egusphere-2025-3404', Anonymous Referee #2, 08 Sep 2025
reply
The manuscript presents the investigation of the volume-to-extinction ratio of dust. In general, this is a very interesting and important topic, however, the manuscript comes short in a proper discussion and explanation of some of its findings and conclusions. Furthermore, to my opinion, the authors are often very imprecise with their explanations/descriptions. Therefore, I suggest major revisions of the manuscript before publication.
General comments:
In the manuscript, two cases are discussed, and the derived volume-to-extinction ratio is compared to former studies. It would be very helpful and valuable to include a detailed characterization of the two cases. To my opinion, the two cases are not directly comparable to former studies of the volume-to-extinction ratio.
The authors should include a better / more detailed discussion about in-situ measurements, especially for the Cape Verde case showing very large values.
The structure of the manuscript is not quite straight forward.
I will now try to give some specific comments to justify my decision:
Line 2: The authors refer to once as volume-to-extinction ratio and once as parameter. It should be made clear that the same is meant.
Line 3: Does the volume really depend on the composition? How?
Line 3: Is combining airborne in-situ and ground-based remote sensing really a novel, synergistic approach? There have been many more studies before doing this.
Line 8: ‘reveal significant variability…’ – Isn’t this depending on the different cases which show differences in the measured properties and rather refer to different mixtures which are reflected by the variability than to a large variability for the same composition.
Line 33: ‘dust can act as both CCN and INP’ – Dust itself (talking about the coarse mode in the mixture as linked to the previous sentence) shows no hygroscopicity and is thus a rather inefficient CCN, except if you refer to dust as a mixture including also the fine mode contribution. But that should be made clearer, especially as you are talking about the coarse mode in the previous part of the section. Give references for your statements.
Line 34: which models? Give references.
Lines 39f: Which various environmental and economic applications? Please give references.
Line 51: Why is the density important for the volume?
Line 56: What is meant by traditional in-situ measurements? To my opinion airborne in-situ measurements are already well-established and provided valuable data also on this topic.
Line 70: To derive the vertical profile, what is the required sample time at one altitude? Please answer in the methodology section.
Lines 75ff: It has to be mentioned that in-situ measurements have to measure the full size-range for improvements. For that, it is also of importance to give a better description of the in-situ measurements in the methodology section.
Line 175f: Did really ESA organize the campaign?
Section 3.2.1 CIMEL: What is the power of the lidar? And is an upper altitude of 30 km really achievable with a cimel lidar? Please give a reference for the system.
Section 3.2.2 PollyXT: Please give information about the lidar instrument that are relevant for this study and describe the performance of the system.
Section 3.3.1 POPS: Please provide information on the measurement principle. How to derive the PSD and the volume? What is the effect of drying the aerosol on the comparability with studies of ambient conditions?
Section 3.3.2 UCASS: Please provide information on the measurement principle. Again, how is the PSD and the volume derived?
Section 3.3.3 Impactors: Please provide information on the measurement principle and what is measured.
For all in-situ measurements, please include a better description of the uncertainties of the in-situ measurements.
Line 282: Why is there trust in the model about this information, when there is little trust in the models in general?
Case selection: More information on the different cases would be helpful, especially for the Cape Verde case with very large reff.
Line 326: What is the criteria for uniform?
Line 328: The VLDR follows the concentration of the aerosol and can thus be misleading. Why do you not use the PLDR for a clearer characterization and classification?
Line 332: The lidar ratio of 35 sr rather indicates that this is not a pure dust case.
Line 334: ‘addressing this uncertainty is beyond the scope of the present study.’ – To my understanding, addressing this uncertainty is essential when the property is used to retrieve the ratio that is claimed to be the main finding of this study! Please give at least an estimate and range for the uncertainty.
Line 335f: Please see my prior comment that the VLDR follows the concentration.
Line 340: What is a sky error?
Line 345: What does balanced contribution mean? A higher contribution of the fine mode does indicate that in this case you don’t have ‘pure’ dust, right?
Figure 4: The main point you want to highlight with this figure should be mentioned in the text.
Figure 5: Same here. Please describe what you want to show with this figure and don’t leave it to the reader to draw their own conclusions.
Equ. 14: Maybe I have missed it, but where do you get the size information from?
Line 383: There have been more studies on the in-situ as well as on the lidar side dealing with that kind of topic. Please give more balanced references of former studies.
Figure 7: Again, please describe the findings. And, it is quite clear that the two cases differ, looking at the information that has been provided so far.
Figure 8: Is the main information of this figure the log-fit? Please specify how the fitting parameters have been derived.
Line 444: There might be also large uncertainties connected to the in-situ measurements. A clear discussion on the uncertainties associated with the in-situ measurements should be included.
Figure 9b: I do not understand how this volume size distribution is derived from the information given in Figure 7. Please provide more explanation.
Line 460: To my understanding, more small particles lead to less volume. This would explain the findings for the case. However, this is not a value for ‘pure’ dust.
Figure 10: The presentation of the findings in this Figure makes it hard or impossible to see what is described in the text. Again, more and detailed information about that case would be very helpful as it is so different to all the other cases. Please also provide an estimate of the uncertainties of the in-situ measurements.
Section 5.2: For my understanding, it would be much easier to follow if this section would be moved to the general description of the volume-to-extinction ratio.
Line 543: How is the total dust mass concentration derived?
Conclusion: It should contain a proper discussion also on the uncertainties of the in-situ measurements. In general, I find it difficult to put such a significance on the findings presented in this study, as the number of cases is not large, and as the largest differences to former studies were found for cases that are, to my opinion, rather not pure dust cases, or where further information would be needed to fully understand what is going on (including a proper error estimation).
Citation: https://doi.org/10.5194/egusphere-2025-3404-RC2
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