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International audience Ambrym volcano (Vanuatu, Southwest Pacific) is one of the largest sources of continuous volcanic emissions worldwide. As well as releasing SO2 that is oxidized to sulfate, volcanic plumes in the troposphere are shown to undergo reactive halogen chemistry whose atmospheric impacts have been little explored to date. Here, two-way nested simulations were performed with the regional scale model CCATT-BRAMS to test our understanding of the volcano plume chemical processing and to assess the impact of Ambrym on atmospheric chemistry at local and regional scales. We focus on an episode of extreme passive degassing that occurred in early 2005 and for which airborne DOAS measurements of SO2 and BrO columns, in the near downwind plume, have been reported. The model was developed to include reactive halogen chemistry and a volcanic emission source specific to this extreme degassing event. SO2 simulated columns show very good quantitative agreement with the DOAS observations as well as with OMI data, suggesting that the plume direction as well as its dilution are well represented. Simulations are presented with and without a high-temperature initialization that includes radicals formed by high temperature partial oxidation of magmatic gases by ambient air. When included high-temperature chemistry initialization, the model is able to capture the observed BrO/SO2 trend with distance from the vent in the near downwind plume. However, the maximum of BrO columns enhancement is still underestimated by a factor 3. The model identifies total in-plume depletion of ozone (15 ppbv) as a limiting factor to the partitioning of reactive bromine into BrO, of particular importance in this very strong plume at low background ozone conditions. Impacts of Ambrym in the Southwest Pacific region were also evaluated. As the plume disperses regionally, reactive halogen chemistry continues on sulfate aerosols produced by SO2 oxidation and promotes BrCl formation. Ozone depletion is weaker than at local scale but still between 10 to 40 %, in an extensive region few thousands of kilometres from Ambrym. The model also predicts transport of bromine to upper troposphere and stratosphere associated with convection events. In the upper troposphere, HBr is re-formed from Br and HO2.
The model confirms the potential for volcanic emissions to influence the oxidizing power of the atmosphere: methane lifetime (calculated with respect to OH and Cl) is overall increased in the model due to the volcanic emissions. Reactive halogen chemistry is responsible for about 62 % of the methane lifetime increase with respect to OH, with depletion of OH by SO2 oxidation responsible for the remainder (38 %). Cl radicals produced in the plume counteract 41 % of the methane lifetime lengthening due to OH depletion. The reactive halogen chemistry in the plume is also responsible for an increase of 36 % of the SO2 lifetime with respect to oxidation by OH. This study confirms the strong influence of Ambrym emissions during the extreme degassing event of early 2005 on the composition of the atmosphere at the local and regional scales. It also stresses the importance of considering reactive halogen chemistry when assessing the impact of volcanic emissions on climate.