Photochemical box modelling of volcanic SO₂ oxidation
Implementing Runge-Kutta algorithms to solve Ordinary Differential Equations at scale to reproduce mass-independent isotopic anomalies in volcanic sulphates, and understand sulphur oxidation pathways in tropospheric plumes.
The photochemical box-model CiTTyCAT is used to analyse the absence of oxygen mass-independent anomalies (O-MIF) in volcanic sulphates produced in the troposphere.
Model Implementation
An aqueous sulphur 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
- H₂O₂, O₃ and O₂ (catalysed by TMI) in the liquid phase
Multiple model simulations are performed 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.
Key Findings
Finding 1: OH carries significant O-MIF
According to model calculations, OH is expected to carry a very significant O-MIF in sulphur-rich volcanic plumes. Hence, the volcanic sulphate produced in the gas phase would have a very significant positive isotopic enrichment.
Finding 2: H₂O₂ is a minor oxidant
Although H₂O₂ is a major oxidant of SO₂ throughout the troposphere, it is very rapidly consumed in sulphur-rich volcanic plumes. As a result, H₂O₂ is found to be a minor oxidant for volcanic SO₂.
Finding 3: O₃ oxidation is negligible
Oxidation of SO₂ by O₃ is negligible because volcanic aqueous phases are too acidic.
Finding 4: O₂/TMI dominance explains isotopic composition
Only cases where sulphur oxidation by O₂/TMI is very dominant can explain the isotopic composition of volcanic tropospheric sulphate.
Published in Atmospheric Chemistry and Physics Read the full paper →