Well constrained volcanic emissions inventories in chemistry transport models are necessary to study the impacts induced by these sources on the tropospheric sulfur composition and on sulfur species concentrations and depositions at the surface. In this paper, the changes induced by the update of the volcanic sulfur emissions inventory are studied using the global chemistry transport model MOCAGE (MOdèle de Chimie Atmosphérique à Grande Échelle). Unlike the previous inventory (Andres and Kasgnoc, 1998), the updated one (Carn et al., 2016, 2017) uses more accurate information and includes contributions from both passive degassing and eruptive emissions. Eruptions are provided as daily total amounts of sulfur dioxide (SO₂) emitted by volcanoes in the Carn et al. (2016, 2017) inventories, and degassing emissions are provided as annual averages with the related mean annual uncertainties of those emissions by volcano. Information on plume altitudes is also available and has been used in the model. We chose to analyze the year 2013, for which only a negligible amount of eruptive volcanic SO₂ emissions is reported, allowing us to focus the study on the impact of passive degassing emissions on the tropospheric sulfur budget. An evaluation against the Ozone Monitoring Instrument (OMI) SO₂ total column and MODIS (Moderate-Resolution Imaging Spectroradiometer) aerosol optical depth (AOD) observations shows the improvements of the model results with the updated inventory. Because the global volcanic SO₂ flux changes from 13 Tg yr⁻¹ in Andres and Kasgnoc (1998) to 23.6 Tg yr⁻¹ in Carn et al. (2016, 2017), significant differences appear in the global sulfur budget, mainly in the free troposphere and in the tropics. Even though volcanic SO₂ emissions represent 15 % of the total annual sulfur emissions, the volcanic contribution to the tropospheric sulfate aerosol burden is 25 %, which is due to the higher altitude of emissions from volcanoes. Moreover, a sensitivity study on passive degassing emissions, using the annual uncertainties of emissions per volcano, also confirmed the nonlinear link between tropospheric sulfur species content with respect to volcanic SO₂ emissions. This study highlights the need for accurate estimates of volcanic sources in chemistry transport models in order to properly simulate tropospheric sulfur species.

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.

The contribution of volcanic emissions is argued as non-linear on the sulfur species burden. Thus, well constraining volcanic emissions inventories is necessary to better study the impacts induced by these pollution sources on the troposheric sulfur composition, as well as on sulfur species concentrations and depositions at the global surface. In this paper, the changes induced by the update of the volcanic sulfur emissions inventory on the global chemistry-transport model MOCAGE (MOdèle de Chimie Atmosphérique à Grande Échelle) are studied. Unlike the current inventory [Andres and Kasgnoc (1998)], the new inventory [Carn et al. (2016, 2017)] includes contributions from both passive degassing and eruptive emissions with more accu- rate information. Eruptions are provided as daily total amounts of sulfur dioxide (SO₂) emitted by volcanoes, while degassing are provided as annual averages with annual uncertainties by volcanoes. Information on plumes altitudes is also available and has been used in the model. The choice is made to look at the year 2013, when a neglieable amount of eruptive volcanic SO2 emissions is referenced, allowing us to focus the study on the impact of passive degassing emissions on the tropospheric sulfur budget. A validation against GOME-2 SO₂ total column and MODIS AOD observations shows the improvements of the model results with the new inventory. Because the global volcanic SO₂ flux changes from 13 Tg.yr⁻¹ in the current inventory to 23.6 Tg.yr⁻¹ in the new inventory, the updated inventory shows significant differences in the global sulfur budget, mainly in the free troposphere and in the tropics. Even though volcanic SO₂ emissions represent 15 % of the total annual sulfur emissions, the volcanic contribution to the tropospheric sulfate aerosol burden is 27 %. Moreover, a sensitivity study on passive degassing emissions, using the annual uncertainties of emissions per volcanoes, also confirmed the non-linear link between tropospheric sulfur species and volcanic emissions. This study highlights the necessity of using accurate estimates of volcanic sources in chemistry-transport models in order to properly simulate tropospheric sulfur species.

Volcanic eruptions are events that can eject several tons of material into the atmosphere. Among these emissions, sulfur dioxide is the main sulfurous volcanic gas. It can form sulfate aerosols that are harmful to health or, being highly soluble, it can condense in water particles and form acid rain. Thus, volcanic eruptions can have an environmental impact on a regional scale.The Mediterranean region is very interesting from this point of view because it is a densely populated region with a strong anthropogenic activity, therefore polluted, in which Mount Etna is also located. Mount Etna is the largest passive SO2 emitter in Europe, but it can also sporadically produce strong eruptive events. It is then likely that the additional input of sulfur compounds into the atmosphere by volcanic emissions may have effects on the regional atmospheric sulfur composition.We are particularly investigating the eruption of Mount Etna on December 24, 2018 [Corradini et al, 2020]. This eruption took place along a 2 km long breach on the side of the volcano, thus at a lower altitude than its main crater. About 100 kt of SO2 and 35 kt of ash were released in total, between December 24 and 30. With the exception of the 24th, the quantities of ash were always lower than the SO2.The availability of the TROPOMI SO2column estimates, at fine spatial resolution (7 km x 3.5 km at nadir) and associated averaging kernels, during this eruptive period made it also an excellent case study. It allows us to follow the evolution of SO2 in the volcanic plume over several days.Using the CNRM MOCAGE chemistry-transport model (CTM), we aim to quantify the impact of this volcanic eruption on atmospheric composition, sulfur deposition and air quality at the regional scale. The comparison of the model with the TROPOMI observation data allows us to assess the ability of the model to properly represent the plume. In spite of a particular meteorological situation, leading to a complex plume transport, MOCAGE shows a good agreement with TROPOMI observations. Thus, from the MOCAGE simulation, we can evaluate the impact of the eruption on the regional concentrations of SO2 and sulfate aerosols, but also analyse the quantities of dry and wet deposition, and compare it to surface measurement stations.

Constraining emission inventories into chemistry-transport models (CTM) is essential. In addition to anthropogenic emissions, natural sources of pollutants must be considered. Among them, volcanoes are large emitters of gases, including sulfur dioxide (SO2), a volatile species, causing environmental and health issues.Volcanic SO2 emission inventories are usually integrated in global CTMs, in order to improve the modelling of chemical species in the atmosphere. Here, we use the model MOCAGE, developed at CNRM, which currently uses Andres & Krasgnoc’s inventory (1998); a temporal average of emission on some 40 volcanoes, monitored through the synergy of satellite data and surface remote sensing instruments, for 25 years (from 1970’s to 1997). However, this inventory is now quite old and is therefore no longer sufficiently accurate.Thanks to the development of new satellite observations, it has become possible to produce such inventories with an improved accuracy. The global coverage and higher sensitivity of these instruments has allowed to reference more emission sources (hard-to-access volcanoes, small eruptions or even passive degassing). Hence, a new inventory of Carn et al (2016,2017) based on satellite observations has been implemented in MOCAGE. Besides being recent (from 1978 up to 2015), it combines eruption and passive degassing over more than 160 volcanoes. Passive degassing fluxes are provided as annual averages and eruption fluxes as daily total quantities (in case of events). In addition, information on volcanoes vent altitude and eruptive plume heights is available, which has been used to better constraints the model.We focus our study at the global scale. The years 2013 and 2014 were chosen as the years with the lowest and highest total eruptive emissions respectively, in Carn's inventory. Thus, 2013 highlights mainly the impact of passive degassing, while 2014 provides additional information on eruptions.For each of the years studied, the sulfur species budget in MOCAGE simulation is increased when the inventory is updated and therefore the relative contribution of volcanic sulfur emissions is larger. We note the global increase in sulfur dioxide and sulfate aerosol burdens; an increase even more significant when the injection heights of the emissions are taken into account.