the Creative Commons Attribution 4.0 License.
the Creative Commons Attribution 4.0 License.
| 24 Sep 2024
In-situ volcanic ash sampling and aerosol-gas analysis based on UAS technologies (AeroVolc)
Abstract. Volcanic degassing and explosive eruptions inject significant amounts of gas and ash into the atmosphere, impacting the local environment and atmospheric dynamics from local to global scales. While ground- and satellite-based remote sensing systems are key to describing explosive volcanism and assessing associated hazards, direct in situ measurements inside volcanic clouds are not possible with these methods. This study presents an innovative approach using an Unoccupied Aircraft System (UAS) for (i) airborne ash sampling and (ii) measurements of aerosol and gas concentrations (AeroVolc system). Commercial instruments (DJITM Matrice 30 Unoccupied Aerial Vehicle (UAV), AlphasenseTM N3 Optical Particle Counter OPC, SoarabilityTM Sniffer4D Mini2 multigas hardware) were combined with custom-built ash collectors and particle counters to enable a more detailed analysis of volcanic clouds. Here we showcase the deployment of our UAS on Sakurajima (Japan) and Etna (Italy), two volcanoes known for their frequent explosive eruptions and persistent degassing activity, to demonstrate how this approach enables in situ, high-resolution sample and data collection within challenging environments. Results provide grain size distributions (GSDs), information on the occurrence of particle aggregation, as well as solid aerosol (PM1, PM2.5, and PM10) and gas (SO2 and CO2) concentrations. Depending on whether the UAS was operated within or below ash- and/or gas-rich clouds, different insights were gained that open up new perspectives for volcanological research. These insights include the composition, concentration, generation, dispersion and sedimentation patterns of volcanic clouds.
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Status: final response (author comments only)
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RC1: 'Comment on amt-2024-162', Anonymous Referee #1, 05 Dec 2024
Review of the manuscript "In-situ volcanic ash sampling and aerosol-gas analysis based on UAS technologies (AeroVolc)".
The idea behind the study is timely and represents a logical next step for using UASs in volcanic environments. The manuscript is well structured and well written. I enjoyed the read and suggest publication with very minor revisions.
I would have liked to read a bit more about the challenges involved during the field campaigns. Did the transmission between remote controller and UAV have issues when flying inside the plume? Was the distance between take off location and measurement area a challenge? Did you encounter corrosion on either the UAV or the sensors after contact with the plume etc..
Detailed comments can be found in the commented pdf.
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AC1: 'Reply on RC1', Simon Thivet, 25 Dec 2024
We thank the anonymous reviewer for its review concerning the paper "In-situ volcanic ash sampling and aerosol-gas analysis based on UAS technologies (AeroVolc)".
We followed point by point the minor corrections and suggestions of the reviewer, and we changed accordingly the manuscript.
We agree and all points and we are grateful to the reviewer for its attentive reading.
Note that 3 references have been added, as suggested, and also to round off the introduction.
Also, and as suggested, we added some points about the challenges involved during the field campaigns (mainly in the method and result sections), which were interestingly very minor (namely only one transmission cut off because of the distance and ground topography, as well as some minor corrosion on the grid of the A2C2/3).
Concerning the comment made on lines 396 to 398, we carefully checked the data pattern displayed by the OPC. It appears that laboratory tests confirmed this pattern as a technical bias, when too many particles are entering the device. This pattern is also not likely at all to explain natural processes such as particle segregation at such scales.
As suggested by the review process, the revised manuscript will be submitted once all reviews will be received and adressed.
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AC1: 'Reply on RC1', Simon Thivet, 25 Dec 2024
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RC2: 'Comment on amt-2024-162', J. Stix, 30 Dec 2024
GENERAL COMMENTS
This paper presents some interesting results of lightweight drone-mounted instrumentation applied to volcanology. I feel that the ash measurements are generally solid and well presented, while the multigas measurements are not robust and poorly presented. My detailed comments follow below.
SPECIFIC COMMENTS
Line 80: Please expand on these generalities. What do you mean by “long” and “far” flights? What do you mean by “payload versatility”? What is the maximum payload weight that the drone can carry? Some details are needed here.
Lines 102-109: Again, some details would be useful here regarding flight times, payload, and batteries. You state that the drone can fly for up to 40 minutes. Did you fly the drone for such long flights? Why or why not? Regarding the batteries, how did you transport these to your field sites? Were you able to bring them on the airplane with you, or did they need to be shipped? This is a very important practical consideration, because shipping of such batteries can be very complicated and expensive. Regarding payload, a maximum weight of 300 grams is very, very small. Why did you use a drone with such small payload capacity? There are many drones today that can easily carry a kilogram or more of payload. So tell the reader why you decided to use this make and model of drone.
Lines 124-125: So how did you fly the drone, manually or autonomously?
Line 204: Does the real-time data transmission include all measured parameters?
Lines 210-211: Some detail is needed here regarding calibration. How often were these calibrations carried out? The gas sensors, particularly the SO2 electrochemical cell, drift with time as they age, thus need to be recalibrated on a regular basis. How are the effects of pressure and temperature dealt with? In particular, CO2 concentrations measured by NDIR sensors are sensitive to pressure and temperature variations, which are of course happening on a drone which is changing altitude very frequently. Last but not least, what is the sensitivity and detection limits of the SO2 and CO2 sensors?
Line 226: The response time of SO2 electrochemical cells is much slower than CO2 NDIR sensors. How did you deal with this issue?
Lines 246-249: In North America, the maximum permissible altitude for a drone under normal conditions is 120 m above ground level. You state that you flew at altitudes of up to 500-1500 m. Did this respect local rules?
Line 257: Of course, volcanic particles are not spherical. Was this an issue?
Lines 364-378: The Sniffer 4D is a potentially very interesting device, because it holds the potential to measure the CO2/SO2 ratio of volcanic gas. This is a crucially important ratio, both for forecasting eruptions and for understanding the volcanic plumbing system, including injections of new CO2-rich magma from deep levels. So what are you telling the reader here? Is the instrument able to make such ratio measurements or not? You state on line 369 that “CO2 concentrations are steady during the entire multigas analysis (between 0.06 and 0.07 %).” This does not make sense to me. Either there should be clear increases of volcanic CO2 above the atmospheric background of ~425 ppm, or else the NDIR sensor is not able to resolve background CO2 from volcanic CO2. Modern multigas instruments can easily measure changes of 1-2 ppm CO2. So it is very important to explain to the reader what is going on here, and whether this instrument can truly be used as a multigas instrument or not under these conditions. My guess is that since the volcanic gas concentrations are pretty low (max SO2 of 1.4 ppm), the CO2 sensor is not able to see and distinguish the volcanic contribution, which is probably only a few ppm higher than the 425 ppm background signal. Is this surmise correct or not?
It is also important to verify the drone-based multigas with a ground-based multigas, to show that the gas ratios, e.g., CO2/SO2 in this case, measured by drone are reliable.
Line 420: How close or far are these aggregates from spherical? How might this affect the accuracy of your particle size measurements?
Lines 493-497: It is worthwhile pointing out that the ability to measure SO2 concentrations means that you can potentially measure SO2 fluxes from a volcano. If the drone is able to reasonably fly perpendicularly through a plume and record SO2 concentrations through this transect, then by knowing the width of the plume (calculated by the drone’s speed and the time spent in the plume) and the plume speed (also potentially measurable by the drone), it is then possible to calculate an SO2 flux.
John Stix
30 December 2024
Citation: https://doi.org/10.5194/amt-2024-162-RC2 -
AC2: 'Reply on RC2', Simon Thivet, 09 Jan 2025
We are grateful for the comments made by the reviewer.
We are pleased that the manuscript was generally well received, and we have improved the parts that were not understood and not detailed enough (especially regarding the gas and CO2 aspects), following the reviewer's comments. We also added a new reference (De Moor et al. 2019 GRL) to support a newly addressed point in the conclusion.The revised version of the manuscript will contain all the new points addressed in the review.
Line 80:
We provide more details in this part. Note that a full description of the drone characteristics is provided in the following paragraphs. Thus, we adapted the structure of this part to make it clearer.Line 102-109:
Thanks for your comment; we have detailed this part following your suggestions. All equipment described in this study can be transported by airplane without any restrictions. This point has been added in the manuscript at the beginning of the section (for each UAV battery, the energy rating is 131.6 Wh, which does not exceed the 160 Wh limit allowed on commercial flights). For the payload capacity, higher capacities indeed exist. However, we selected this specific drone because our scientific objectives required only a modest payload capacity (under 300 grams). Consequently, larger drones, capable of carrying kilograms of equipment, would have offered no real advantage while introducing unnecessary weight, complexity, and cost. Moreover, the chosen drone’s other characteristics—such as flight endurance, ease of use/transportation, and overall reliability—made it well-suited to our project’s needs. We explain the choice of the drone, based on multiple criteria, in section 2.1.Line 124-125:
Thanks for noticing that. The drone was piloted manually to adapt to any environmental changes during volcanic activity. This point has been added to the manuscript in section 2.3.Line 204:
We added some details on this point (we have access to PM, SO2, and CO2), which is already good for gaining insights into the plume intersection.Line 210-211:
Thanks for your comment. We have added more details about the multigas instrument in the manuscript. It was originally designed to be carried by this UAV and is calibrated every year. As you said, these sensors are exposed to data drifting over time. Thus, the instrument is checked and calibrated by the manufacturer on an annual basis, as recommended (using proper calibration chambers for all sensors). As explained in the method section and by the manufacturer of this instrument (which was originally designed for drone measurements), changes in atmospheric conditions are directly taken into account by the device to mitigate and compensate for the effects of P, T, and RH. We clarified this point in the manuscript. Of course, we have to avoid sudden jumps in T, P, and RH as much as possible to be as precise as possible. Also, the flight shown in this study using the multigas device was vertically fixed (stable attitude), thus with stable P (cf. supplementary file S7 for an exhaustive data description acquired by the multigas). Detection limits and sensitivities are also well described in the manuscript (For CO2: detection limits between 0.01 (100 ppm, which is also the sensitivity) and 5%, with a max deviation of 2%; for SO2: detection limits between 0.000075% (750 ppb, also the sensitivity) and 0.01%, with a maximum measurement deviation of 4%). This has been clarified in the method.Line 226:
We had a specific concern about this point. In fact, the device automatically synchronizes the data (on-chip proprietary algorithm), taking into account the different response times of the sensors. As an example, we can see in the multigas data that PM and SO2 are perfectly correlated in time (peaks and variations occur at the same time, apart from the few natural exceptions described in the manuscript, where peaks of PM occur independently of the gas concentrations), which confirms that data are synchronized considering the different response time of the different sensors.Lines 246-249:
Indeed, regulations vary depending on the location, country, etc., and we are careful about that. Field campaigns were performed at Sakurajima and Etna. At Sakurajima, flight regulations allowed flying 500 m above the takeoff point (because of the relative proximity of two airports). At Etna, this limit was increased to 1500 m. Regulations are always checked on the online international database, which is regularly updated on the drone.Line 257:
Of course, we have to make this approximation, which in the end is not impactful for this kind of measurement, especially for these fine grain size fractions (fine particles with high aspect ratios). This approximation is widely assumed for OPC measurements.Line 364-378:
We totally agree with your comment. In the presented case, the CO2 sensor is not able to detect variations (which are probably only a few ppm above background) because it is not sensitive enough to detect such small changes in gas concentration (the sensor resolution is 100 ppm). This is a limitation of this device’s CO2 sensor. However, as you also mentioned in your comment, higher CO2 fluxes in some cases could be detectable. We clarified this point in the manuscript, specifically in the discussion section.
Regarding the CO2/SO2 ratio: indeed, this is an important parameter to consider, and we developed that point in the perspectives, as we were not able to measure it properly. We also specified the possibility (and necessity) of upgrading the wide-range CO2 sensor (100 ppm resolution) to a more precise (1 ppm) sensing module that can be installed on the payload instrument. That will be the next upgrade to allow reliable CO2/SO2 measurements on specific targets (we also add the reference of De Moor et al. 2019 to support this perspective). Note that the multigas measurements were also performed at ground level and show similar patterns to the data acquired at airborne levels, though with lower gas and PM concentrations because the volcanic plume was very diluted at these levels.Line 420:
Aggregates are close to being spherical and do not affect the overall GSD. Note that all particle sizes reported in the analysis are equivalent spherical diameters. We add this point in the method section. Also, as explained in the perspectives, the next step of image analysis is to count all the particles inside and outside the aggregates to have a real and total GSD, with the information on the number of particles inside and outside the aggregates. This analysis is currently being carried out for another study focused on the ash aggregation mechanism.Line 493-497:
Thanks for your suggestion. We have added this point in the perspectives.Citation: https://doi.org/10.5194/amt-2024-162-AC2
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AC2: 'Reply on RC2', Simon Thivet, 09 Jan 2025
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