Volcanoes are emitters of gaseous species (H2O, CO2 and sulfur compounds, halogen hydracids (HBr, HCl)) and aerosols into the troposphere from passive emissions and moderate volcanic eruptions, which can cause ozone loss. Modeling studies have explored this phenomenon, primarily focused on the first few hours after emission and more localized analyses. Jourdain et al. (2016) conduced the only study at a regional scale for a case of strong passive degassing at Ambryn.

This paper shows results of a new modelling study at the regional scale with the chemistry-transport model MOCAGE, focused on the Mt Etna volcanic eruption that occurred around Christmas 2018 and that lasted 6 days. The authors test the ability of the regional 3D CTM MOCAGE model to simulate the bromine-explosion cycle, to analyze the different chemical processes in the volcanic plume at different distances from the vent and to quantify its impact on the tropospheric composition at the regional scale. In other words, they analyze the variability of the chemical processes during the plume transport and quantify its impact on the composition of the troposphere at a regional scale over the Mediterranean basin.

Great paper, indeed, as it fulfills the mandates and deliverables directed by Centre National de Recherches Météorologiques of Météo-France, PREV’AIR program (Rouil et al., 2009) for France, and in the framework of the Copernicus project for Europe (Marécal et al., 2015), and for monitoring volcanic eruptions as part of the Toulouse VAAC (Volcanic Ash Advisory Center) of Météo-France – all of which converge on providing accurate and precise forecasts/model simulations of climate and air quality. Within this context, I believe this manuscript should be published in ACP. Given the substantive dataset collected and reduced, I recommend provided overarching/broader context of the implications your data – basically, over the course of the Eruption, was the air quality adversely impacted? If so/not, to what extent, quantitatively/qualitatively – especially climate and air quality @ Earth’s surface/within the boundary layer in those regions. Moreover, were their any associated human health/ecosystem/build-environment impacts associated w/ the Eruption? I also suggest considering juxtaposing Figures 2 and 3 somehow as it would be great to see them next to each other to compare TROPOMI satellite column SO2 and BrO profiles and MOCAGE model simulated column BrO and SO2 profiles. Also, the degree of congruence appears hard to interpret, especially for BrO as TROPOMI column profiles exhibit a background of BrO throughout the region (w/ slight variability in the region) and 6 days of Eruption while MOCAGE shows slight variability in distinct places w/ no excess BrO background (as shown in the TROPOMI BrO profiles). Atop this, is it possible to quantify the degree of congruence of the MOCAGE model results w/ TROPOMI satellite profiles?

The manuscript presents results from a modeling study of an Etna volcanic eruption that occurred in December 2018 and lasted approximately 1 week. The objective of this study is to demonstrate and evaluate the ability of a 3D regional chemical transport model (MOCAGE) to simulate volcanic halogen chemistry. Furthermore, the impact on tropospheric composition on a regional scale over the Mediterranean region is assessed. The authors then compare the simulated tropospheric SO2 and BrO columns with those derived from TROPOMI satellite measurements and find good agreement when the uncertainties in the flux estimates and TROPOMI columns are taken into account. By analyzing the distribution of the different bromine species and the associated chemical reaction rates, the authors of this comprehensive study provide a detailed picture of the simulated bromine chemistry during the diurnal cycle and at different stages of plume development. The authors conclude that the distribution of bromine species is mainly influenced by the time evolution of emissions during the eruption, meteorological conditions, and the distance of the plume from the vent or the time since emission. Finally, the authors discuss the consequences of the obtained results in terms of the impact of the volcanic eruption on the OH and O3 oxidants at the regional scale, concluding here that the reduced concentrations lead to a decrease in the atmospheric oxidation capacity.

The effects of volcanic emissions may have important contributions to chemistry on a global scale and even more so on a regional scale. Understanding these and describing them in models is therefore important and relevant to atmospheric chemistry. The paper is written in great detail, which limits readability, but obviously offers advantages to researchers who also wish to develop such models. I therefore endorse the publication in ACP with a few minor suggested changes.

Page 2, last sentence of abstract: I don't know all the results of previous volcanic halogen modeling but the statement that the results are consistent with all previous ones seems a bit strange.

Page 2, line 31: "in the form of SO2" (there are different sulfur species but no different SO2 species)

Page 2, line 35: “magmatic gases” instead of “magmatic air” pp

Page 11, line 255: Which other cycles are meant here ?

Page 11, line 268: “….and are thus individually represented as diagnostic species” ??

Page 13, line 335: Certainly not important and just a detail: In an earlier part of the manuscript the height of Mt Etna is given as 3330 m – 50 m above would mean 3380m.

Page 20, line 439: I would not use abbreviations in the main text => molecules cm-2 s-1