the Creative Commons Attribution 4.0 License.
the Creative Commons Attribution 4.0 License.
| 31 Jan 2025
A Novel Simplified Ground-Based TIR System for Volcanic Plume Geometry, SO2 Columnar Abundance, and Flux Retrievals
Abstract. In the last few decades, volcanic monitoring using remote sensing systems has become an essential tool to investigate the effects of volcanic activity on environment, climate, human health and aviation, as well as to give insights into volcanic processes. Compared to satellite measurements, ground-based instruments offer continuous spatial and temporal coverage capable of providing high resolution and high sensitivity data.
This work presents a new simplified prototype of a Thermal InfraRed (TIR) system (named “VIRSO2”). The instrument comprises three cameras, one working in the visible and two in the TIR (8–14 μm). In front of one of the two TIR cameras, an 8.7 μm filter is placed. The system is designed for detection of volcanic emission, geometry estimation, columnar content of SO2 and ash, and SO2 flux retrievals. The retrieval procedures developed are detailed starting from the geometric characterization with wind direction correction, the calibration by considering the effects of filter multireflections and temperature, and the SO2 mass by exploiting MODTRAN radiative transfer model (RTM) simulations. The SO2 flux is then computed by applying the traverse method, with the plume speed obtained from the wind speed at the crater altitude. As test cases, the measurements collected at Etna volcano (Italy) on the 1 April 2021 during a lava fountain episode and the 30 August 2024 during a quiescent phase have been considered. The results show that the system can provide reliable information on plume detection, altitude, and SO2 flux.
The simplicity, low cost, and the possibility of carrying out measurements at a safe distance from the vent both day and night, make this system ideal for real-time monitoring of volcanic emissions, thus helping to provide information on the state of activity of the volcano and therefore to mitigate the effect that these natural phenomena have on humans and the environment.
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Status: open (until 02 Apr 2025)
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RC1: 'Comment on egusphere-2025-63', Hugues Brenot, 20 Feb 2025
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Review of "A Novel Simplified Ground-Based TIR System for Volcanic Plume Geometry, SO2 Columnar Abundance, and Flux Retrievals"
This paper presents a comprehensive methodology for retrieving SO2 column density using a ground-based TIR system (2 IR cameras and one visible camera). The study is well-structured, detailing the three key steps: computing the instrument view geometry and the preparation of the data from both IR cameras (one broad band and one narrow band with a filter at 8.7µm), calibration (using the radiative transfer model called MODTRAN), and SO2 retrieval (using look-up tables from MODTRAN). The authors provide an analysis of error propagation and uncertainty quantification, making the findings valuable for the atmospheric and volcanology communities. Overall, this is a well-executed study that presents a useful methodology for SO2 flux retrievals, with a balanced discussion of its advantages and limitations.
Suggestions of technical corrections:
Line 37: "but are punctual" → "but are limited to specific locations"
Lines 50-51: "and as part of a continuous and real-time volcanic monitoring system." → ", and as part of a continuous, real-time volcanic monitoring system."
Lines 77-78: "manufacturer supplied" → "manufacturer-supplied"
Line 80: Why is the conversion into brightness temperature performed at 10.02 µm?
Figure 2a: The quality of the text box could be improved.
Line 145: In the legend of Figure 4, replace "m2" with "m²".
Lines 162-166: Equations 9, 10, 11, and 12 are not easy to conceptualise. To help the reader, a new illustration could be added to clearly show x(j) and explain mx and qx. Alternatively, additional text could be included to clarify these equations.
Line 169: In Equation 3.8, does the reference correspond to Equation 8?
General question regarding Section 3.1: How is the wind direction (ω) determined? Is it estimated during the field campaign, or is it derived from GEO (SEVIRI) or LEO (TROPOMI) data? If this is the case, it should be explicitly stated.
Lines 180-184: SEVIRI retrievals estimate a plume top altitude of 6.9 km at 00:55 UTC on April 1, 2021. You state that this is in good agreement with the VIRSO2 nighttime measurements. Could you specify the volcanic plume top altitude obtained using VIRSO2’s field of view and geometric considerations, both with and without wind correction?
Lines 190-193: The error increases for ω > 45°. Could you provide an explanation for this? Additionally, what conclusions can be drawn from the comparison between this study and the tool provided by Snee et al. (2023)?
Lines 200-202: The text mentions "the next section," but the order of presentation is Sections 4.1, 4.3, and 4.2. Consider reordering the text to follow the sequence 4.1, 4.2, and 4.3, or, if this is not logical, switching the order of Sections 4.3 and 4.2.
Lines 200-202: "at first, the non-perfect transmissivity of the 8.7 µm filter produces a 'ghost image.' Then, the filter temperature affects the NB measurements, and finally, an adjustment is necessary for the BB camera, considering that the clear sky temperature often does not match the MODTRAN simulations." The mention of "non-perfect transmissivity" and "filter temperature effects" for the NB measurements appears closely related. Additionally, some introductory information about the use of MODTRAN would be beneficial. Improving the clarity of these three lines would help the reader grasp the concept more easily.
Line 206: The phrase "do not frame exactly the same scene" describes a common issue known as an X/Y shift between the two cameras. This terminology could be included for clarity.
Line 213: "For the BB camera, this effect is small enough that a correction is not required (Prata et al., 2024)." Given that the vignetting effect results in a range of 4 K for the NB and only 0.4 K for the BB, it seems reasonable to assume that vignetting correction is not necessary for the BB camera.
Lines 230-232: "A 'correction image' (Fig. 7c) is simply obtained as the difference between the black target temperature image Tn,NB(i, j) and its mean value MEAN(Tb,NB) over the 320 × 240 array." It is unclear why MEAN(Tb,NB) is subtracted from Tn,NB(i, j). Could you provide an explanation? Is this a way to normalise the ghosting effect? What is the purpose of applying this offset? Clarifying this would be helpful.
Line 287: In Equation 15, it appears that the term B is not explicitly defined.
Line 306: In Figures 8a and 8b, using the same Y-axis range (e.g., 210–290 K) could improve visual consistency.
Line 313: In Section 4.3, you wrote, "as described in Sect. 4." Did you mean "As described in the introduction of Section 4"?
Lines 322 & 331: To fully understand Equations 16 and 17, it is crucial to define the term B.
Line 351: In Section 4.3, you wrote, "as already reported in Sect. 4." Please specify whether this refers to the introduction of Section 4, Section 4.1, or Section 4.2.
Lines 350-355: It would be helpful to explicitly define TBB(sky), TBB(ground), T'NB(sky), and T'NB(ground).
Line 374: In Figure 10a, there still appears to be some vignetting effect. If confirmed, this should be mentioned.
Line 401: Equation 22 does not seem to match Figure 11. The equation states:
BH = D0 tan(θ)
However, if BMP represents the mean altitude of the MODTRAN plume layer above sea level, the correct expression should be:
BMP = (D0 + HT/2) tan(θ) + h0
BMP should also be added to Figure 11.
Line 415: It may be useful to remind the reader that TS represents the clear sky temperature.
Line 429: In Figure 13a, there still appears to be some vignetting effect. If confirmed, this should be mentioned.
Citation: https://doi.org/10.5194/egusphere-2025-63-RC1
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