Comment on gmd-2022-180

Abstract

<strong class="journal-contentHeaderColor">Abstract.</strong> Volcanoes are a known source of halogens to the atmosphere. HBr volcanic emissions lead rapidly to the formation of BrO within volcanic plumes. BrO, having a longer residence time in the atmosphere than HBr, is expected to have an impact on tropospheric chemistry, at least at the local and regional scales. The objective of this paper is to prepare a framework for further 3-D modelling of volcanic halogen emissions in order to determine their fate within the volcanic plume and then in the atmosphere at the regional and global scales. This work is based on a 1-D configuration of the global chemistry transport model MOCAGE whose low computational cost allows us to perform a large set of sensitivity simulations. This paper studies the Mount Etna eruption on 10 May, 2008. Several reactions are added to MOCAGE to represent the halogen chemistry occurring within the volcanic plume. A simple sub-grid scale parameterization of the volcanic plume is also implemented and tested. The use of this parameterization tends to limit slightly the efficiency of BrO net production. Both simulations with and without the parameterization give similar results for the partitioning of the bromine species, ozone depletion and of the BrO / SO₂ ratio that are consistent with previous studies and with the BrO / SO₂ ratio in the volcanic plume estimated from GOME-2 spaceborne observations. A series of test experiments were performed to evaluate the sensitivity of the results to the composition of the emissions, and, in particular, primary sulphate aerosols, the Br radical, and NO. Simulations show that the plume chemistry is sensitive to these assumptions. Another series of tests on the effective radius assumed for the volcanic sulphate aerosols shows that BrO net production is sensitive to this parameter with lower BrO concentrations reached when larger aerosols (smaller total surface area) are assumed. We also find that the maximum altitude of the eruption changes the BrO production, which is linked to the vertical variability of the concentrations of oxidants. These sensitivity tests display changes in the bromine chemistry cycles that are generally at least as important as the subgrid scale plume parameterization. Overall, the version of the MOCAGE chemistry developed for this study is suitable to produce the expected halogen chemistry in volcanic plumes during daytime and night. These results will be used to guide the implementation of volcanic halogen emissions in the 3-D configuration of MOCAGE for regional and global simulations.