Photochemical box modelling of volcanic SO₂ oxidation: isotopic constraints

Abstract. The photochemical box model CiTTyCAT is used to analyse the absence of oxygen mass-independent anomalies (O-MIF) in volcanic sulfates produced in the troposphere. An aqueous sulfur oxidation module is implemented in the model and coupled to an oxygen isotopic scheme describing the transfer of O-MIF during the oxidation of SO₂ by OH in the gas-phase, and by H₂O₂, O₃ and O₂ catalysed by TMI in the liquid phase. Multiple model simulations are performed in order to explore the relative importance of the various oxidation pathways for a range of plausible conditions in volcanic plumes. Note that the chemical conditions prevailing in dense volcanic plumes are radically different from those prevailing in the surrounding background air. The first salient finding is that, according to model calculations, OH is expected to carry a very significant O-MIF in sulfur-rich volcanic plumes and, hence, that the volcanic sulfate produced in the gas phase would have a very significant positive isotopic enrichment. The second finding is that, although H₂O₂ is a major oxidant of SO₂ throughout the troposphere, it is very rapidly consumed in sulfur-rich volcanic plumes. As a result, H₂O₂ is found to be a minor oxidant for volcanic SO₂. According to the simulations, oxidation of SO₂ by O₃ is negligible because volcanic aqueous phases are too acidic. The model predictions of minor or negligible sulfur oxidation by H₂O₂ and O₃, two oxidants carrying large O-MIF, are consistent with the absence of O-MIF seen in most isotopic measurements of volcanic tropospheric sulfate. The third finding is that oxidation by O₂∕TMI in volcanic plumes could be very substantial and, in some cases, dominant, notably because the rates of SO₂ oxidation by OH, H₂O₂ and O₃ are vastly reduced in a volcanic plume compared to the background air. Only cases where sulfur oxidation by O₂∕TMI is very dominant can explain the isotopic composition of volcanic tropospheric sulfate.