Modelling volcanic sulphate formation in the troposphere: volcanic halogens role and isotopic constraints

International audienceVolcanic emissions are an important source of atmospheric pollutants. Notably, sulfate aerosols formation from volcanic sulfur emissions induce regional climatic impacts, such as: "vog" production, acid deposition, and hazardous effects on health. Large uncertainties are still pertaining to tropospheric volcanic sulfate production, as active sulfur oxidation pathways within a plume are still uncertain. Recent observations indicate that volcanic emissions are characterized also by large concentrations of reactive halogens. Notably, halogens affect atmospheric oxidants budget via ozone depletion events (ODEs), thus influencing potentially sulfur oxidation in a volcanic plume. A photochemical box-model CiTTyCAT is used to simulate sulfur oxidation within the core of volcanic plumes, and resulting sulfate oxygen isotopic composition. The model accounts for SO2 oxidation by OH in the gas phase, and by H2O2, O3, O2/TMI, and HOX (HOX = HOBr + HOCl) in the liquid phase of: either water droplets and water on ash (referred as condensing plume), or sulfate aerosols in absence of ash (referred as non-condensing plume). It also describes transfer of oxygen mass-independent anomalies (O-MIF) from oxidants to produced sulfate. For condensing plumes, ODEs may also occur in presence of water droplets, while not affecting the hierarchy of oxidation pathways. Sulfur oxidation by O2/TMI remains dominant even for low TMI concentrations, reproducing most isotopic measurements of tropospheric sulfates extracted from volcanic ash, whose O-MIFs equal to 0 ‰. For non-condensing plumes ODEs occur even at very low halogens loading (ca. 50 ppbv). Sulfur oxidation rate is much lower than for condensing plumes, while being driven by H2O2 on sulphate aerosols, and by OH in the gas phase. Sulfate production and relative H2O2 and OH contributions to oxidation depend on halogen loading: larger halogens concentrations induce higher H2O2 contribution, resulting in lower sulfur oxidation. For non-condensing plumes sulfate O-MIF is between 0.8 and 1.4 ‰, in contrast to O-MIF absence in tropospheric sulfates extracted from volcanic ash. These results suggest that sulfate found on tropospheric volcanic ash-deposits is likely formed via sulfur oxidation in water-rich phases, and not via oxidation in the gas-phase or within sulfate aerosols