In this study we present a pre-operational forecasting assimilation system of different types of aerosols. This system has been developed within the chemistry-transport model of Météo-France, MOCAGE, and uses the assimilation of the Aerosol Optical Depth (AOD) from MODIS (Moderate Resolution Imaging Spectroradiometer) onboard both Terra and Aqua. It is based on the AOD assimilation system within the MOCAGE model. It operates on a daily basis with a global configuration of $1^{\circ }\times 1^{\circ }$ (longitude × latitude). The motivation of such a development is the capability to predict and anticipate extreme events and their impacts on the air quality and the aviation safety in the case of a huge volcanic eruption. The validation of the pre-operational system outputs has been done in terms of AOD compared against the global AERONET observations within two complete years (January 2018–December 2019). The comparison between both datasets shows that the correlation between the MODIS assimilated outputs and AERONET over the whole period of study is 0.77, whereas the biases and the RMSE (Root Mean Square Error) are 0.006 and 0.135, respectively. The ability of the pre-operational system to predict extreme events in near real time such as the desert dust transport and the propagation of the biomass burning was tested and evaluated. We particularly presented and documented the desert dust outbreak which occurred over Greece in late March 2018 as well as the wildfire event which happened on Australia between July 2019 and February 2020. We only presented these two events, but globally the assimilation chain has shown that it is capable of predicting desert dust events and biomass burning aerosols which happen all over the globe.

In this study, we focus on the eruption of Mount Etna on Christmas 2018, which emitted great amounts of $\mathrm{S}{\mathrm{O}}_2$ from 24th to 30th December into the free troposphere. Simulations based on two different estimations of $\mathrm{S}{\mathrm{O}}_2$ emission fluxes are conducted with the chemistry-transport model MOCAGE in order to study the impact of these estimations on the volcanic plume modeling. The two flux emissions used are retrieved (1) from the ground-based network FLAME, located on the flank of the volcano, and (2) from the spaceborne instrument SEVIRI onboard the geostationary satellite MSG. Multiple spaceborne observations, in the infrared and ultraviolet bands, are used to evaluate the model results. Overall, the model results match well with the plume location over the period of the eruption showing the good transport of the volcanic plume by the model, which is linked to the use of a realistic estimation of the altitude of injection of the emissions. However, there are some discrepancies in the plume concentrations of $\mathrm{S}{\mathrm{O}}_2$ between the two simulations, which are due to the differences between the two emission flux estimations used that are large on some of the days. These differences are linked to uncertainties in the retrieval methods and observations used to derive $\mathrm{S}{\mathrm{O}}_2$ volcanic fluxes. We find that the uncertainties in the satellite-retrieved column of $\mathrm{S}{\mathrm{O}}_2$ used for the evaluation of the simulations, linked to the instrument sensitivity and/or the retrieval algorithm, are sometimes nearly as large as the differences between the two simulations. This shows a limitation of the use of satellite retrievals of $\mathrm{S}{\mathrm{O}}_2$ concentrations to quantitatively validate modeled volcanic plumes. In the paper, we also discuss approaches to improve the simulation of $\mathrm{S}{\mathrm{O}}_2$ concentrations in volcanic plumes through model improvements and also via more advanced methods to more effectively use satellite-derived products.

Volcanoes are known to be important emitters of atmospheric gases and aerosols, which for certain volcanoes can include halogen gases and in particular HBr. HBr emitted in this way can undergo rapid atmospheric oxidation chemistry (known as the bromine explosion) within the volcanic emission plume, leading to the production of bromine oxide (BrO) and ozone depletion. In this work, we present the results of a modelling study of a volcanic eruption from Mt Etna that occurred around Christmas 2018 and lasted 6 d. The aims of this study are to demonstrate and evaluate the ability of the regional 3D chemistry transport model Modèle de Chimie Atmosphérique de Grande Echelle (MOCAGE) to simulate the volcanic halogen chemistry in this case study, to analyse the variability of the chemical processes during the plume transport, and to quantify its impact on the composition of the troposphere at a regional scale over the Mediterranean basin. The comparison of the tropospheric SO₂ and BrO columns from 25 to 30 December 2018 from the MOCAGE simulation with the columns derived from the TROPOspheric Monitoring Instrument (TROPOMI) satellite measurements shows a very good agreement for the transport of the plume and a good consistency for the concentrations if considering the uncertainties in the flux estimates and the TROPOMI columns. The analysis of the bromine species' partitioning and of the associated chemical reaction rates provides a detailed picture of the simulated bromine chemistry throughout the diurnal cycle and at different stages of the volcanic plume's evolution. The partitioning of the bromine species is modulated by the time evolution of the emissions during the 6 d of the eruption; by the meteorological conditions; and by the distance of the plume from the vent, which is equivalent to the time since the emission. As the plume travels further from the vent, the halogen source gas HBr becomes depleted, BrO production in the plume becomes less efficient, and ozone depletion (proceeding via the Br+O₃ reaction followed by the BrO self-reaction) decreases. The depletion of HBr relative to the other prevalent hydracid HCl leads to a shift in the relative concentrations of the Br⁻ and Cl⁻ ions, which in turn leads to reduced production of Br₂ relative to BrCl. The MOCAGE simulations show a regional impact of the volcanic eruption on the oxidants OH and O₃ with a reduced burden of both gases that is caused by the chemistry in the volcanic plume. This reduction in atmospheric oxidation capacity results in a reduced CH₄ burden. Finally, sensitivity tests on the composition of the emissions carried out in this work show that the production of BrO is higher when the volcanic emissions of sulfate aerosols are increased but occurs very slowly when no sulfate and Br radicals are assumed to be in the emissions. Both sensitivity tests highlight a significant impact on the oxidants in the troposphere at the regional scale of these assumptions. All the results of this modelling study, in particular the rapid formation of BrO, which leads to a significant loss of tropospheric ozone, are consistent with previous studies carried out on the modelling of volcanic halogens.

Sulfur dioxide emitted during volcanic eruptions can be hazardous for aviation safety. As part of their activities, the Volcanic Ash Advisory Centres (VAACs) are therefore interested in the real-time atmospheric monitoring of this gas. A recent development aims at improving the forecasts of volcanic sulfur dioxide quantities made by the MOCAGE (Modèle de Chimie Atmosphérique à Grande Échelle) chemistry transport model. For this purpose, observations from both TROPOMI (Tropospheric Monitoring Instrument) and IASI (Infrared Atmospheric Sounding Interferometer; B and C) located on separate polar-orbiting satellites are assimilated into the model. These sulfur dioxide measurements are based on the eruption event of the La Soufrière Saint Vincent volcano in April 2021. Observations from OMI (Ozone Monitoring Instrument) are considered validation data. The resulting assimilation experiments show that the combined assimilation of IASI and TROPOMI observations always leads to a better forecast compared to the independent assimilation of data from each instrument. Sulfur dioxide atmospheric field forecasts are better when the available observations are numerous and cover a long time window.

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.

Sulfur dioxide emitted during volcanic eruptions can be hazardous for aviation safety. As part of their activities, the Volcanic Ash Advisory Centres (VAACs) are therefore interested in the real-time atmospheric monitoring of this gas. A recent development aims at improving the forecasts of volcanic sulfur dioxide quantities made by the MOCAGE chemistry transport model. For this purpose, observations from both TROPOMI and IASI (B and C) instruments located on separate polar orbiting satellites are assimilated in the model. These sulfur dioxide measurements are based on the eruption event of the La Soufrière Saint-Vincent volcano in April 2021. Observations from the OMI instrument are considered as validation data. The resulting assimilation experiments show that the combined assimilation of IASI and TROPOMI observations always leads to a better forecast compared to the independent assimilation of data from each instrument. Sulfur dioxide atmospheric field forecasts are better when the available observations are numerous and cover a long time window.