the Creative Commons Attribution 4.0 License.
the Creative Commons Attribution 4.0 License.
| 08 May 2026
Volcanic sulfur dioxide total columns and layer height from the IASI and TROPOMI Layer Height products: preprocessing and assimilation on a 0.1° regional domain of MOCAGE
Abstract. Sulfur dioxide (SO2) emitted by volcanic activity constitutes an important hazard for aircraft operations. Volcanic Ash Advisory Centres (VAACs) rely on satellite observations to monitor and on the assimilation of these observations to predict the evolution of volcanic plumes. TROPOMI (Tropospheric Monitoring Instrument) observations are already operational in the MOCAGE (Modèle de Chimie Atmosphérique à Grande Échelle) model. However, in this MOCAGE configuration, SO2 plumes are assumed to be located between 3 and 10 km of altitude. Recent developments aim to improve SO2 modelling by taking into account plume altitude data provided by the TROPOMI Layer Height (TROPOMI LH) product and the Infrared Atmospheric Sounding Interferometer (IASI) in the assimilation. Before being assimilated, these observations are preprocessed according to two methods depending on the assimilated instrument.
This approach is evaluated for two contrasting eruptions: the explosive eruption of Mount Etna on 4 August 2024 and the low-altitude eruption on the Reykjanes Peninsula between 22 and 25 August 2024. Independent observations from the Ozone Mapping and Profiler Suite (OMPS) and AirBase surface measurements are used for validation. The results show that the use of plume altitude improves the vertical representation of SO2 and facilitates the detection of high SO2 total columns. Despite the limited availability of altitude observations, their use improves plume structure and transport consistency.
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Status: final response (author comments only)
- RC1: 'Comment on egusphere-2026-2098', Anonymous Referee #1, 01 Jul 2026
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RC2: 'Comment on egusphere-2026-2098', Anonymous Referee #2, 30 Jul 2026
Review of “Volcanic sulfur dioxide total columns and layer heights from the IASI and TROPOMI Layer Height products: preprocessing and assimilation on a 0.1-degree regional domain MOCAGE” by Bacles and Guidard
The paper present an evaluation of different approaches to assimilating volcanic observations into the MOCAGE offline chemical transport model. The goal is to improve the simulation of volcanic SO2 amount and altitude to help in situational awareness (prediction) during volcanic events. Two case studies are evaluated, and several assimilation approaches are tried: simple TROPOMI SO2 amount, TROPOMI SO2 and layer height, IASI SO2 and layer height, and a joint approach that combines TROPOMI SO2/layer height with IASI. Preprocessing steps are described that are meant to homogenize the observations for suitability in assimilation into MOCAGE. Results are compared (somewhat qualitatively) with OMPS and MSG satellite observations and AirBase surface observations. Including layer height information in the assimilation generally resulted in higher simulated SO2 abundances than SO2 amount alone, which is a positive outcome and showed ability to simulate observed surface concentrations for low altitude plume events.
The paper is generally well-structured and well-written. Most of my comments are for improving clarity of presentation, but I do raise a couple of issues that I think warrant further revision. First, the preprocessing approaches described do not seem clearly justified and seem to result in—for lack of a better word—invention of observations that I don’t find entirely credible. More discussion on the preprocessing is therefore warranted. Second, MOCAGE itself seems to struggle with data retention (i.e., SO2 added is removed very quickly) which is evident especially in Figure 10, which is still somewhat at odds with the high SO2 values returned. It seems the authors think that plume thickness assumptions are partly the culprit, but I think some further information about the model chemical mechanism is needed (i.e., is SO2 rapidly scavenged or otherwise converted to sulfate?). Finally, I think that this paper needs to also present the analysis increments for the various simulations (graphically and numerically) to show where and how SO2 increments are applied.
Line 56: Was not familiar with AirBase. Please just briefly describe here what that is.
Line 90: It is probably beyond the scope of this study, but how is the “case of polluted scenarios” determined from the observations?
Line 178: Why are you not using IFS fields to drive the model, consistent with the chemical boundary conditions in the previous paragraph?
Line 294: What is the rationale for the approach in this algorithm? It seems to aggressively elevate points that are at reasonable altitudes (8-9 km in Figure 3c). Does this make sense?
Line 310: You explain why you don’t want to use large horizontal correlation lengths, but I wonder if there is an intermediate length that could be tried. The interpolation approach to me seems to invent a lot of data where there is none, which I find problematic. I just can’t tell that the high SO2 going into Turkey is real in Figure 6b.
Line 422: There seems to be a real data retention issue in the model. What is happening to this SO2? Is it being scavenged? Rapidly converted to sulfate? How does MOCAGE compare to other aerosol models in this respect? Too aggressive?
Line 430: Maybe, but as noted you seem to also have data retention issues so I’m skeptical. What are your analysis increments? Suggest you add a plot that shows that.
Line 455: What dates are being shown in Figure 11? The times do not clearly correspond to, e.g., what is shown in Figure 7. Oh, I see, this is confusing. 04AM means morning on the 4th, etc.
Line 496: The opening sentence seems not correct. Yes is higher than IASI on 04AM, Tropomi_lh is lest on 05AM. What do you mean to say?
Line 580: Caption to Figure 15 mentions other colored lines which I don’t see and don’t appear to be described in the text.
Citation: https://doi.org/10.5194/egusphere-2026-2098-RC2
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Please find the review in the attachement.